1. Field of the Invention
The present invention relates to filter and amplifier circuits for current signals, and in particular, to filter and amplifier circuits for current signals which use resistive-capacitive (RC), low-pass filter circuits implemented in integrated circuit (IC) form.
2. Description of the Related Art
In many circuits involving mixed signals (i.e., analog plus digital), it is often to necessary to include filter circuits to prevent switching noise or higher order signal frequency components from the digital signals from affecting the analog signals. This is particularly true for ICs which are processing both analog and digital circuits on one chip.
As ICs have become larger and more complex, the need for including filter circuits has grown greater due to the proximity of analog and digital circuits and signal lines, as well as the use of shared power supply lines within the chip. One of the simpler types of filter circuits used is a low-pass filter circuit which relys on resistors and capacitors. An implementation of an RC-type low-pass filter is typically done in one of two ways. One technique is to provide terminals for connecting external capacitors to the chip. However, this technique requires dedicated terminals for connecting the external capacitors. For many complex ICs, this is unacceptable due to the limited number of external pins or other terminals available for a particular size of IC package. The second technique involves the implementation of the filter using on-chip integrated components. For example, it is well know to use a metal oxide semiconductor field effect transistor (MOSFET), with its drain, source and substrate terminals all connected together, as a capacitor.
Referring to FIG. 3, however, using a MOS transistor as a capacitor requires that some form of stable voltage bias be provided to as to establish and stabilize the capacitance value C of such capacitor. For example, to maintain operation of the MOS transistor in its depletion mode, the drain/source/substrate terminal must be biased positively with respect to its gate terminal with a DC bias voltage Von which is greater than the transistor threshold voltage Vt. Alternatively, the DC bias voltage Von must be maintained at a negative value less than zero volts at the gate terminal with respect to the drain/source/substrate terminal to maintain operation in the accumulation mode.
Further, once the capacitance value has been stabilized in this manner, it must be maintained under signal conditions. Accordingly, it becomes necessary to combine the input AC signal with the DC biasing signal, thereby resulting in an output signal which contains both AC and DC components. However, this is generally undesired and it often becomes necessary to then somehow remove the DC signal component from the output signal.
Accordingly, it would be desirable to have a simple and reliable technique whereby on-chip integrated components can be used to implement an RC-type filter having stabilized values of capacitance while being capable of amplifying an AC input signal and providing a corresponding AC output signal with no DC components corresponding to any internal DC biasing signal.